1. Field of the Invention
The present invention relates generally to vascular grafts and methods of implantation of vascular grafts, and in particular, to graft systems or assemblies for use in grafting and methods for deploying these graft systems in bypass grafting procedures.
2. Description of the Prior Art
Over the past thirty years, a large number of vascular grafts have been surgically implanted in patients to (i) revascularize blood flow from diseased arteries and veins, (ii) to replace the diseased arteries and veins, and (iii) to bypass regions of severe stenosis. These vascular grafts have been provided in the form of autogenous grafts, synthetic grafts, or grafts of biological origins (homogeneous or heterogeneous). Synthetic grafts are generally used for mending large arteries, while autogenous saphenous veins are generally used for arterial reconstruction of smaller vessels (such as in the lower extermities). In aortocoronary bypass, autogenous vein grafts are typically anastomosed proximally to the ascending aorta and distally to the coronary artery downstream from the stenosis.
Occlusion of implanted grafts leading to graft failure is a major problem encountered in all cases. In general, in the case of coronary arterial bypass grafts (CABG), the patency rates of human saphenous vein grafts varies, but by ten years, only fifty percent of such implanted grafts are expected to remain patent, with about half of the patent grafts suffering from severe atherosclerosis. Unfortunately, the patency rate is even lower for grafts used in lower-extremity bypass cases.
The causes of such graft failures can be categorized as intrinsic and extrinsic factors. Intrinsic factors involve the adaptations within the graft wall itself, and intimal hyperplasia and atherosclerosis are two major intrinsic factors associated with post-operative failure of arterial bypass grafts. Extrinsic factors involve physiology related to, but not directly part of, the graft, such as the blood, the arterial bed into which the graft is placed, and the surgical technique.
In CABG applications, early occlusion (i.e., less than one month from the surgical procedure) occurs in about five to fifteen percent of all cases. In fact, graft occlusion within one week of the procedure occurs in about seven to eight percent of the cases. These numbers are significant, and attempts have been made to minimize the percentage of early occlusion cases by (i) utilizing techniques of surgical preparation that preserve a nonthrombogenic endothelium, and (ii) providing an optimal anastomosis.
Optimal anastomosis is especially important to the potential patency of the graft, but a number of factors make it difficult to achieve. For example, the graft opening must be properly sized to prevent kinking at the anastomotic site. Meticulous (i.e., careful) anastomosis is also required for small anastomosis to obtain good patency. However, these procedures may be difficult to accomplish if the vein graft is collapsed. To address this problem, a pickup forceps is typically used to hold the vein opening during aortocoronary and peripheral vascular bypass surgery. However, the forceps may cause endothelial injury and can slip from its position during the anastomosis. Other holding devices (e.g., the Mobin-Uddin vein graft holder) were developed to address the deficiencies of the forceps, but these devices are still not completely satisfactory when used for anastomosis because they may still cause injury to the endothelium, or do not provide satisfactory circumferential support to the vein graft.
Optimal anastomosis should minimize the occurrence of bleeding. In many cases, bleeding can be a problem after completion of the anastomosis. Although a significant etiologic factor for this bleeding is systemic heparinization or an acquired platelet dysfunction associated with cardiopulmonary bypass, the surgical site may also be a contributing factor because of suboptimal surgical techniques used during the anastomosis. The proximal anastomosis of an aortocoronary artery bypass graft is one such potential site. Factors that may contribute to bleeding at this site include the quality of the aorta and implanted saphenous vein, as well as the anastomotic stitch spacing and tension. However, one factor that is particularly troublesome in some cases is that the aortotomy is significantly larger in size than the diameter of the saphenous vein (i.e., there is a size mismatch). In such cases, the wall of the vein becomes stretched and tensioned at the proximal anastomotic site, as shown in FIG. 3B (which is described in greater detail hereinbelow). The size mismatch also results in a tendency for the anastomotic suture to cut through the vein, resulting in bleeding. Additional stitches placed to control this bleeding may result in further tearing of the vein, thereby exacerbating the condition so that the proximal anastomosis becomes a site of major hemorrhage. In addition to the bleeding problems, a mismatch in the size of the aortotomy and the saphenous vein graft may cause the vein to flatten at the site of the anastomosis, thereby impairing blood flow through the graft.
The optimal way to manage this difficult mismatch situation would be to avoid it by appropriately judging the size of the aortotomy. However, it is very difficult to properly predict how much an anastomosed vein graft will expand when subject to arterial pressure. Thus, this mismatch in the size of the aortotomy and the saphenous vein graft will occur in many cases. If significant bleeding results from such a mismatch or other unsatisfactory vein contour, a number of surgical options are available. According to one option, the vein graft can be disconnected from the aortotomy (which is then closed), and the proximal end of the vein graft is refashioned and anastomosed to a more appropriately-sized aortotomy. According to another option, the vein graft can be disconnected from the aortotomy (which is then closed), and the vein is anastomosed in end-to-side manner to another saphenous vein graft that has already been joined to the aorta. Unfortunately, there are potential problems with these two surgical options. An aortotomy, especially an oversized one, can be difficult to close hemostatically. In addition, a direct vein-to-vein anastomosis where one of the veins is markedly narrow may potentially place both grafts at risk for early occlusion. As a result, a third surgical option is to place a partial-thickness stitch circumferentially around the aortotomy. A partial-thickness stitch does not extend entirely through the wall of the aortotomy, and the stitch has to be tied with sufficient tension to reduce the circumference of the aortotomy but without cutting through the aorta. Unfortunately, partial-thickness stitches may cause the layers that make up the aortotomy to separate (known as delamination).
In addition to minimizing bleeding, an optimal anastomosis should also provide (i) proper anastomotic geometry (e.g., opening, inflow and outflow tracts) to ensure a smooth rheologic boundary, and (ii) minimal internal wall stress in connecting vessels or grafts at the anastomotic region.
Regarding proper anastomotic geometry, it is important to note that materials with different mechanical properties (also known as this compliance mismatch), when joined together and placed in a cyclic stress system, exhibit different extensibilities. Compliance mismatch can be defined as the nominal difference in compliance between the blood vessel and a synthetic graft. "Extensibility" describes how much a vessel or a graft expands under arterial pressure. Stress concentrations at or near the site of coaptation can result in marked changes of geometry (e.g., out-of-plane bending, and buckling).
Regarding internal wall stress, it should be noted that compliance mismatches may cause increased stress at the anastomotic sites, as well as create flow disturbances and turbulence. Suture lines can also cause additional local compliance mismatches at the connection of the graft and the vessel, and may affect how stress is transmitted to an anastomotic site. It is believed that compliance mismatches at the interface of the graft and the vessel causes regional hemodynamic disturbances, which result in turbulent blood flow and shear forces that are imparted to adjacent flow surfaces. Such flow disruption may lead to para-anastomotic intimal hyperplasia, anastomotic aneurysms, and the acceleration of downstream atherosclerotic change.
Thus, there still remains a need for a graft assembly or system which promotes optimal anastomosis, which distributes stresses in an optimal manner, which is easy to implant, and which generally minimizes or avoids the problems described hereinabove.